Proposed changes to a key inflation measure have been thrown open to consultation amid fears that pension savers could be hit hardest.

The Office for National Statistics (ONS) will request views on changes to the calculation of the retail prices index (RPI) rate of inflation, which could lead to RPI moving more slowly and in step with the consumer prices index (CPI).

RPI usually rises faster than CPI, with an average gap of 0.9% each year since CPI was first used in 1996, so a slowdown would directly depress future increases in private sector pensions.

The consultation comes after the Government earlier this year uprated the calculation of public sector pensions from RPI to CPI.

Darren Philip, policy director at the National Association of Pension Funds, said changes to the way RPI is constructed would have "huge implications". He said: "Pension funds are major investors in government debt and changes to index-linked bonds could have far-reaching impacts on those investments.

"It could also alter the amount by which pensions being paid to former workers are increased each year."

As well as private sector pensions, RPI changes would affect the value of inflation-linked savings certificates and index-linked bonds, or gilts, issued by the Government. But rail passengers could benefit from a slower-rising RPI rate as around half of all fares are linked by a government formula to July's RPI inflation rate.

The differences between the RPI and CPI under consideration are those caused when different formulae are used to calculate average prices where there is no information about precise spending.